1. Field
The invention is in the field of dimmers for, and methods of, variably dimming fluorescent lights.
2. State of the Art
Continuous dimming of electrical lamps of all types can be desirable for a number of reasons. It can change the atmosphere of a room or, from a more practical standpoint, the power to the lamps of a lighting system can initially be reduced to a desired illumination level to save energy and then increased during use of the system up to full value to compensate for illuminance losses caused by lamp lumen depreciation, dirt effects, and other light loss factors. It is also desirable sometimes to adjust the illumination output of a system depending on the varying natural light conditions.
The dimming of fluorescent lamps present problems because upon start-up of a fluorescent lamp, a large voltage is required to turn the lamp on by initiating the electrical arc in the tube. However, once started, the lamp's resistance decreases and if the current to the arc is not limited, the lamp would draw excessive current damaging various components of the electrical supply system and possibly causing the lamp to explode. The current regulation is generally accomplished by the standard inductive ballasts used with most fluorescent lamps which provides two windings, one for the high voltage arc discharge portion of the lamp and a separate winding for the lower voltage filament portion of the lamp. The high voltage winding has an air gap which limits current flow in the lamp. The filament winding is a normal transformer winding without air gap.
Phase control dimmers where energy to the load is controlled by varying the firing angle or "on" time of each half cycle of the A.C. supply power are commonly used to dim incandescent lamps. However, while such dimmers when used in conjunction with standard fluorescent lamps using standard lamp ballasts can satisfactorily control the energy to the high voltage portion of the lamps, at low levels of applied power (large A.C firing angles), it does not provide sufficient power to the filaments of the lamps and it has been found that fluorescent lamp life is materially shortened. Therefore, phase control dimmers are not used for fluorescent lamp dimming.
Fluorescent lamp dimmers currently available generally use special ballasts for the lamp so that the filament is supplied its required power while power, either the voltage or current, or both are reduced to the discharge portion of the lamp to control brightness. A major drawback of all dimming systems using special ballasts is that all of the normal ballasts have to be replaced with the special ballasts in order to dim the lamps involved. This can get very expensive.
It has been suggested that gas discharge lamps, such as the standard fluorescent lamps, may be dimmed by varying the frequency of the power to the lamps. The basis of such dimming is that as the frequency is increased, the power transferred through the air gap in the high voltage winding of the ballast decreases. Since the filament portion of the ballast has no air gap, it is not frequency sensitive so the power supplied to the filament remains at an acceptable level. This type of dimming system has the problem that variable frequency controls are expensive and inefficient. It also produces a high level of noise when switching transistors are used.
A need still exists for a dimmer for fluorescent lighting systems wherein the standard ballasts can be used eliminating the expense of new electronic ballasts, and which provides good dimming control over a wide illumination range while maintaining the filament voltage at an acceptable level.